Blood coagulation is a process consisting of a complex interaction of various blood components or factors which eventually gives rise to a fibrin clot. Generally, the blood components which participate in what has been referred to as the coagulation "cascade" are proenzymes or zymogens, enzymatically inactive proteins which are converted to proteolytic enzymes by the action of an activator, itself an activated clotting factor. Coagulation factors which have undergone such a conversion are generally referred to as "active factors," and are designated by the addition of a lower case postscript "a" (e.g., factor VIIa).
There are two separate systems which promote blood clotting and which thereby participate in normal hemostasis. These systems have been referred to as the "intrinsic" and the "extrinsic coagulation pathways. " The "intrinsic pathway" refers to those reactions which lead to thrombin formation through utilization of factors present only in plasma. An intermediate event in the intrinsic pathway is the activation of factor IX to factor IXa, a reaction catalyzed by factor XIa and calcium ions. Factor IXa then participates in the activation of factor X in the presence of factor VIIIa, phospholipid, and calcium ions.
The "extrinsic pathway" involves plasma factors, and additionally involves components present in tissue extracts. Factor VII, one of the proenzymes referred to above, participates in the extrinsic pathway of blood coagulation by converting (upon its activation to VIIa) factor X to Xa in the presence of tissue factor and calcium ions. Factor Xa in turn converts prothrombin to thrombin in the presence of factor Va, calcium ions, and phospholipid.
In some instances, for example, kidney dialysis, deep vein thrombosis, and disseminated intravascular coagulation (DIC), it is necessary to block the coagulation cascade through the use of anticoagulants, such as heparin, coumarin, derivatives of coumarin, indandione derivatives, or other agents. For example, a heparin treatment or an extracorporeal treatment with citrate ion (U.S. Pat. No. 4,500,309) may be used in dialysis to prevent coagulation in the course of treatment. Heparin is also used in preventing deep vein thrombosis in patients undergoing surgery. Treatment with low doses of heparin may, however, cause heavy bleeding. Furthermore, because heparin has a half-life of approximately 80 minutes, it is rapidly cleared from the blood. Because heparin acts as a cofactor for antithrombin III (AT III), and antithrombin III is rapidly depleted in DIC treatment, it is often difficult to maintain the proper heparin dosage, necessitating continuous monitoring of AT III and heparin levels. Heparin is also ineffective if AT III depletion is extreme. Further, prolonged use of heparin may also increase platelet aggregation and reduce platelet count, and has been implicated in the development of osteoporosis. Indandione derivatives may also have toxic side effects.
In addition to the anticoagulants briefly described above, there are a variety of compositions disclosed within the art which are alleged to have anticoagulant activity. One such composition is disclosed by Reutelingsperger et al. (Eur. J. Biochem. 151: 625-629, 1985), who isolated a 32,000 dalton polypeptide from human umbilical cord arteries. Another composition is disclosed by Warn-Cramer et al. (Circulation Suppl, part 2, 74: 2-408ii, Abstract #1630, 1986). They detected a factor VIIa inhibitor of an apparent molecular weight of 34,500 in plasma. In addition, Broze et al. (Circulation Suppl, part 2, 74: 2-409ii, Abstract #1634, 1986) studied the inhibition of tissue factor by serum. Their results suggest that barium-absorbed serum contains a moiety which inhibits the factor VIIa-Ca++-tissue factor complex. However, none of these compositions has been well characterized.
Consequently, there is still a need in the art for improved compositions having anticoagulant activity which do not produce the undesirable side effects associated with traditional anticoagulant compositions. The present invention fulfills this need, and further provides other related advantages.